1. Field of the Invention
The invention relates to hydraulic forming of a tubular component under high pressure compressive conditions.
2. Description of the Related Art
In the context of hydraulic forming of a tubular component it is known to place a tubular component into an initially open forming tool comprised of an upper die and a lower die, to fill it with a liquid forming medium, and to seal the ends of the component by means of sealing mandrels. After closing the forming tool, a hydraulic pressure is generated in the component in order to form the component to the preset contours within the forming tool.
This process is based on a hydraulic closed press which is designed with respect to control considerations such that during the hydroforming process the press closes the forming tool for an extended period of time. The closing duration, for example, for vehicle components such as longitudinal beams and transverse supports, is in the range of 5 to 10 seconds. This results in cycle times in the range of approximately 30 to 40 seconds for producing each finished part, this duration including the time required for introducing the component to be formed into the forming tool as well as for the removal of the formed component from the forming tool.
It is an object of the present invention to provide a device for hydraulic forming of a tubular component or a blank under high pressure compressive conditions which, while providing a problem-free integration into a travel-limited mechanical press, is constructively simple and easy to handle.
In accordance with the present invention, this is achieved in that, in the device for hydraulic forming of a tubular component under high inner pressure compressive conditions in a lower die and an upper die of a forming tool, the upper die can be coupled for a relative limited movement by at least one piston-cylinder unit containing a hydraulic fluid with the press plunger of a travel-limited mechanical press and in that the cylinder interior (cylinder chamber) of the piston-cylinder unit can be coupled with the interior of the component for providing fluid communication.
In accordance with the present invention, this is also achieved in that, in the device for hydraulic forming of a blank under high pressure compressive conditions in a forming tool comprising a lower die and an upper die, the upper die can be coupled for a relative limited movement by at least one piston-cylinder unit containing a hydraulic fluid with the press plunger of a travel-limited mechanical press and in that the cylinder interior of the piston-cylinder unit can be coupled with the forming space of the forming tool for providing fluid communication.
The invention combines in an advantageous manner a forming tool with lower die and upper die for hydraulic forming of tubular components under high inner pressure compressive conditions or for hydraulic forming of blanks under high pressure compressive conditions with a travel-limited mechanical press known, for example, in the form of an eccentric press, crank press or knuckle joint lever press.
A characteristic feature of such a mechanical press is a continuous movement performed in a continuous operation. In contrast, high pressure forming occurring during the forming process requires a completely closed forming tool with a closing force that is sufficiently great over the time period of the forming step. These two contrary conditions are reconciled by the invention in that the upper die of the forming tool can be coupled by at least one piston-cylinder unit containing hydraulic fluid with a press plunger of a mechanical press so as to be moveable to a limited extent relative to the press plunger. As a result of such a configuration with a piston-cylinder unit, a quasi hydraulic cushion between the upper die and the press plunger is generated. This hydraulic cushion then allows a decoupling of the continuous plunger movement from the forming tool for the time period of the high pressure forming action on a tubular component or a blank in the range of the bottom dead center position. At the same time, the piston-cylinder unit is employed in order to use the fluid contained in the piston-cylinder unit directly for forming the component or blank. For this purpose, the cylinder of the piston-cylinder unit is connectable with the interior of the component or with the forming space of the forming tool for providing or establishing fluid communication. As a result of this, a separate pressure intensifier as well as corresponding hydraulic apparatus and components can be omitted.
The advantages of the configuration according to the invention reside in a substantial reduction of the cycle times, a reduction of the investment costs for the components required for pressure generation, as well as a considerably reduced control expenditure. Moreover, this results in the great advantage that in a manufacturing facility the already present capacities of mechanical presses can now be used for the high pressure forming especially of small batch numbers of tubular components or blanks to be shaped or formed.
Depending on the type and contour course of the respective tubular component or of a blank to be formed, only one piston-cylinder unit or several piston-cylinder units are introduced between the upper die and the press plunger. In particular, the arrangement of several smaller piston-cylinder units along a component or blank contour and a direct hydraulic connection, wherein the piston surface corresponds to the projected surfaces of component or the blank, can ensure that at any moment of the forming process a force equilibrium between the piston-cylinder unit and the component or the blank is present. In this arrangement, the further advantage is realized that the elastic deformations in the forming tool can be minimized which provides an improved manufacturing precision.
The filling of a tubular component can be realized with conventional hydraulic apparatus. However, conceivable is also a variant in which filling of the component is carried out in an immersion tank.
Since the piston-cylinder unit is formed as a separate device that can be detached from the upper die as well as the press plunger, it can be used with a flexible configuration for different forming tools and mechanical presses.
An advantageous further embodiment of the invention resides in that the piston of the piston-cylinder unit can be detachably fastened by means of a piston plate to the press plunger and the cylinder can be detachably fastened by means of a cylinder plate to the upper die. The piston plate, moreover, has return members fixedly arranged thereat which are connected to the cylinder or the cylinder plate so as to be relatively moveable. The return members can be, for example, guide rods which penetrate consoles on the cylinder or the cylinder plate so as to be movable relative to the cylinder or cylinder plate. They are provided at their free end with engaging heads which engage from below the consoles or the cylinder plate and, upon upward movement of the press plunger, lift the cylinder or the cylinder plate and thus the upper die.
When a line is provided between the piston-cylinder unit and the forming tool and a fluid separator is integrated in the line, in the area of the piston-cylinder unit a hydraulic oil, optionally with suitable additives, can be advantageously used and for forming the tubular component or the blank an aqueous fluid with only minimal lubricant additives can be used. In this connection, it may also be expedient to design the fluid separator as a pressure intensifier with only a minimal intensifying ratio. As a result of this, additional free spaces are available for designing and adapting the piston-cylinder unit relative to a tubular component or a blank.
In order to ensure during forming a complete adaptation of a tubular component or of a blank to the forming space, the piston-cylinder unit is designed such that the volume displaced by it is greater than that which is required for forming. In order to receive this excess volume, the line between the piston-cylinder unit and the forming tool is provided with a pressure reducer with pressure control valve arranged downstream. The pressure control valve provided at the low pressure side of the pressure reducer has the effect that, only when a certain inner pressure is reached in the component or in the forming space, the volume displaced by the piston-cylinder unit is removed via the pressure reducer. This preset pressure corresponds to the pressure which is required for the complete filling of the forming space by the component or the adaptation of the blank to the contours of the forming space. Such a solution of the pressure or volume limitation is advantageous as a result of the very high pressures occurring during hydroforming in the range of 600 bar to 3,000 bar because for this application a direct pressure limitation by commercially available pressure regulators is not possible.